This invention relates to electrolytic cells and a method of operating such cells. More particularly, this invention relates to electrolytic cells of the membrane-type and a method of tensioning the membrane used in such cells.
Electrolytic cells of the "membrane-type" are well known in the art. The membrane-type cells employ a membrane as opposed to a diaphragm to separate an anode compartment containing anolyte from a cathode compartment containing catholyte. The membranes employed are usually sheet-like and made of material, such as fluoropolymer ion-exchange material which are capable of transporting electrolyte ions while being substantially hydraulically-impermeable. Diaphragms on the other hand, which are usually made of asbestos, permit limited, but substantial, flow through of aqueous electrolyte solution.
Electrolytic cells employing membranes include, for example, monopolar and bipolar type cells including filter press cells. U.S. Pat. Nos. 4,108,742 and 4,111,779, for example, describe typical bipolar cells. The bipolar cell generally includes repeating units of an anode compartment with an anode therein and cathode compartment with a cathode therein separated by a membrane. The electrolytic cells above are typically employed for the electrolysis of aqueous salt solutions, for example, alkali metal chloride brines used in the production of chlorine and alkali metal hydroxides.
When ion-exchange membranes are employed in membrane-type electrolytic cells, it is advantageous to keep the membrane stretched and taut between the electrode compartments to provide a uniform flat membrane surface and minimize wrinkling of the membrane. Wrinkles in the membrane can form because the membranes tend to absorb water and thus swell a certain percentage after they are installed in a cell and contacted with electrolyte at cell operating conditions. The amount of swelling depends on the amount of water in the electrolyte. Swelling of the membrane causes the membrane to lose its tension or tautness between the electrode compartment in a cell which then causes wrinkling to occur on the membrane. Wrinkles formed on a membrane surface can trap gas bubbles, such as chlorine and hydrogen, during an electrolytic process, for example, in the production of chlorine and caustic. These trapped bubbles increase the electrical resistance and power consumption of a cell and, thus, reduce the efficiency of the cell.
In addition to the problem of trapping gas bubbles, a loose or expanded membrane may vibrate excessively between the electrode compartments during operation of the cell and eventually may damage the membrane due to mechanical wear. It is also known that wrinkles are sites for the initiation of cracks in the membrane. Cracks permit leakage of electrolyte between the cell compartments which result in contamination of the cell products and reduced cell performance. Cracks can also occur on the barrier layer of a multi-layered membrane which can allow back migration of ions, such as OH.sup.- migration into the catholyte causing loss of cell efficiency.
In an attempt to solve the membrane wrinkling problem described above, methods have been used to pretreat or precondition the membrane prior to inserting it between the electrode compartments in a cell. For example, the membrane described in U.S. Pat. No. 4,376,030 is presoaked in a solvent and then air dried prior to its installation in a cell to minimize the swelling of the membrane, and thus reduce the slack and wrinkling during actual operation of the cell. It is, however, desired to further reduce the slack and wrinkling of membranes used in the membrane-type electrolytic cells.